1. Field of the Invention
The present invention relates to a highly stable photoreceptor having high sensitivity and stably producing quality images even when repeatedly used, and to an image forming apparatus and a process cartridge therefore using the photoreceptor.
2. Discussion of the Related Art
Recently, developments in information processing systems using electrophotography have proceeded rapidly. In particular, laser printers and digital copiers converting information into digital signals with light have been dramatically improved in terms of print quality and reliability. The laser printers and digital copiers now so widespread are required to reliably produce high-quality images at high speed.
Electrophotographic photoreceptors using organic photosensitive materials are widely used in image forming apparatuses because of the advantages they offer in terms of cost, productivity, environmental stability, etc.
In particular, azo pigments and phthalocyanine pigments are effectively used as charge generation materials for use in organic photoreceptors. As the azo pigments, Japanese published unexamined applications Nos. 47-37543 and 52-55643 disclose benzidine bisazo compounds; Japanese published unexamined application No. 52-8832 discloses stilbene bisazo compounds; Japanese published unexamined application No. 58-222152 discloses diphenylhexatriene bisazo compounds; and Japanese published unexamined application No. 58-222153 discloses diphenylbutadiene bisazo compounds.
Phthalocyanine includes a titanyl phthalocyanine pigment, a metal-free phthalocyanine pigment, a hydroxy gallium phthalocyanine pigment, etc. Specific examples of the titanyl phthalocyanine pigment include an α-type disclosed in Japanese published unexamined publication No. 61-239248, a Y-type disclosed in Japanese published unexamined publication No. H01-17066, an I-type disclosed in Japanese published unexamined publication No. 61-109056, an A-type disclosed in Japanese published unexamined publication No. 62-67094, a B-type disclosed in Japanese published unexamined publications Nos. 63-364 and 63-366, a B-type disclosed in Japanese published unexamined publication No. 2005-15682, a m-type disclosed in Japanese published unexamined publication No. 63-198067, and a semi-amorphous type disclosed in Japanese published unexamined publication No. H01-123868, etc. Specific example of the metal-free phthalocyanine pigment include a X-type metal-free phthalocyanine disclosed in U.S. Pat. No. 3,357,989, a τ-type metal-free phthalocyanine disclosed in Japanese published unexamined publication No. 58-182639, etc. Specific examples of the hydroxy gallium pigment are disclosed in Japanese published unexamined publications Nos. 5-263007 and 5-279591. Specific examples of copper phthalocyanine pigments are disclosed in Japanese published unexamined publications Nos. 58-100134, 61-273994, 62-62367, etc. Specific examples of chlorophthalocyanine pigments are disclosed in Japanese published unexamined publications Nos. 59-44053, 1-221459, etc. Specific examples of chloroindiumphthalocyanine pigments are disclosed in Japanese published unexamined publication No. 60-59355, etc.
However, a photoreceptor using a single pigment of these suffices neither for sensitivity nor stability in usage environment and over time, and does not suffice for future high-quality or high-speed copier photoreceptor. In order to solve this problem, use of combinations of two or more pigments is known. For example, Japanese published unexamined publications Nos. 5-301292 and 2001-290296 disclose a combination of a metal-free phthalocyanine pigment and fluorenone azo pigment. Japanese published unexamined publication No. 9-127711 discloses a combination of a phthalocyanine compound and an azo pigment. Japanese published unexamined publication No. 2002-23399 discloses a combination of a metal phthalocyanine and a perylene pigment. Japanese published unexamined publication No. 2007-334099 discloses a combination of a quinacridone pigment and a titanylphthalocyanine pigment. Japanese published unexamined publication No. 3-9962 discloses a combination of a titanylphthalocyanine pigment and another phthalocyanine pigment. However, even these do not suffice, because the above-mentioned pigments typically have very low solubility in organic solvents and are difficult to uniformly disperse therein, and moreover can be purified only with organic solvents and thus impurities are thought not to be fully removed.
The present inventor discloses an azo pigment having improved solubility in an organic solvent in Japanese published unexamined publication No. 2009-7523, so that a photoreceptor can be prepared with a uniformly-dispersed coating liquid.
With a single charge generation material it is difficult to prepare a photoreceptor having sufficient properties, and therefore a combination of two or more charge generation materials can be contemplated.
Since mechanical means such as milling are mostly used to mix plural pigments they cannot be mixed or combined on the molecular level, and thus the organic photoconductors are thought not to have attained their full functions. In addition, methods of using an acid paste together are disclosed, but a concentrated sulfuric acid used therein is a problem in mixing and possibly decomposes as a pigment, resulting in usage limitation. Further, crystal transformation is made or a desired crystal form is not obtained when mixed with an acid paste, resulting in a photoreceptor having insufficient properties.
Accordingly, a good charge generation material is desired to achieve an electrophotographic photoreceptor having none of the conventional defects.
Because of these reasons, a need exists for a high-sensitive electrophotographic photoreceptor preventing charge accumulation therein due to a charge generation material to produce high-quality images without image defects at high speed for long periods.